A variety of multichannel RF signal distribution systems currently are being employed to deliver commercial broadcast television programming to residential customers. These RF transmission systems often are called "wireless cable" television systems, because they can provide multichannel entertainment programming identical to conventional cable television services, but without the cost and disruption incurred in installing video cable between the program provider's studio and each customer's residence.
United States electronic equipment suppliers have manufactured RF transmission systems to provide Multichannel Multipoint Distribution Service (MMDS) as authorized by the United States Federal Regulations Title 47 (Telecommunication). These MMDS systems have been installed in major metropolitan areas and are used by the television entertainment industry to augment conventional television broadcasts by transmitting premium videos to residential subscribers on a fee (pay-per-view) basis. MMDS uses allocated spectrum at various frequencies in the 2.1 to 2.7 GHz band to transmit fourteen independent channels of video. The MMDS transmitters are installed at locations authorized by the United States Federal Communications Commission (FCC). Each of these transmitter locations has been selected so that it can broadcast into the surrounding service area without creating interference in the adjacent service areas.
In responding to the need for additional wireless multipoint television distribution spectrum (i.e. in addition to the authorized MMDS spectrum), the FCC issued an interim operating license in the 27.5 to 29.5 GHz band. The technology employed for use of this spectrum has been designated Local Multipoint Distribution Service (LMDS) and one implementation of a LMDS is disclosed in U.S. patent application Ser. No. 4,747,160. Both LMDS and its predecessor MMDS broadcast multichannel television signals into specified "service areas". Service areas (also referred to as "cells") identify non-overlapping geographic regions that receive interference-free transmission from separate transmitter sites.
Another prior art television broadcast technology similar to LMDS is designated Millimeter-wave Multichannel Multipoint Video Distribution Service (M.sup.3 VDS) and is described in detail in 1989 IEEE MTT (pages 1095-1102).
Because all of these systems have similar configurations and because they employ related technologies, it is useful to regard these wireless multichannel television RF broadcast systems as implementations of the same system concept. Henceforth, these systems (i.e. MMDS, LMDS, M.sup.3 VDS, and any similar system) will be referred to as multipoint distribution systems (MDS).
Referring to FIG. 1, an MDS typically includes a program provider site 10, a plurality of service area broadcast transmitter towers 11, and a plurality of subscribers 12 in each service area. The program provider distributes multiple channels of signals (via satellite, cable, point-to-point microwave transmission, or fiber optics or any other transmission medium) to each of the service area transmitter towers. Each tower, in turn, broadcasts via RF transmission the received signals (commonly analog signals) to a plurality of subscribers residing in the vicinity surrounding each given tower (i.e., the service area). The range of signal transmission for a given tower (and consequently the size of the service area) is primarily dependent on the power characteristics of the signal being transmitted by the given tower. Each subscriber within a given service area uses an antenna and receiver unit coupled to a television to receive and view the television signals broadcast from the transmitter tower within the given service area. Selection of the desired television channel and adjustment of the audio and video parameters is performed at the television set in the subscriber site by means of controls located on the television or by a remote control unit.
Presently, MDS systems transmit multiple channels of television signals from the service area broadcast towers in parallel. In other words, each distinct television channel (having a given modulation bandwidth) is individually and simultaneously transmitted. Thus, the total bandwidth of the broadcast signal is equal to the sum of the modulation bandwidths of each of these channels plus any additional spectrum used as spacing between the channels to minimize mutual interference.
As an example of this spectrum utilization, MMDS television broadcast systems in the United States employ amplitude modulation (AM) methods which require 6 MHz of bandwidth for each television channel. Up to fourteen of these 6 MHz channels are broadcast by MMDS systems. In contrast, LMDS television broadcast systems in the United States currently employ frequency modulation (FM) methods which require 20 MHz of bandwidth for each television channel. The LMDS system employs 1 GHz of spectrum which enables it to broadcast up to fifty of the 20 MHz wide channels. In the United Kingdom, the M.sup.3 VDS television broadcast system employs FM methods which require 38 MHz of bandwidth for each channel. Consequently, the M.sup.3 VDS system required 304 MHz of spectrum to broadcast the eight television channels initially specified by the British television broadcast authorities.
The modulation methods employed in the above prior art MDS systems completely consume the available spectrum while broadcasting conventional television programming. However, the current trend in cable television technology is to install additional channels so that special video can be provided to interested subscribers. Potentially, these additional channels would be used in special applications such as video-on-demand and video conferencing for selected subscribers. In addition,these channels might be used for computer and data retrieval tasks, for access to the Internet and other data bases, and for interactive applications such as video games and home shopping. These special applications which are targeted at individual subscribers are sometimes designated "narrowcasts" to differentiate them from the usual entertainment television programming that is "broadcast" to all subscribers. Narrowcasting requires many individual and independent channels so that many individual subscribers can be served simultaneously.
Another drawback of presently implemented MDS systems results from signal interaction between adjacent service areas. Specifically, subscribers in vicinities residing along the boundary of a service area may receive signals from the desired service area transmitter tower and also from one or more adjacent service area transmitter towers. These multiple signals entering the subscriber receiver equipment often result in significant degradation of the desired signal quality. Thus, one important design consideration of an MDS transmission system is to ensure that each subscriber receives a strong interference-free signal.
One prior art MDS system designed to avoid mutual interference problems among adjacent service areas (disclosed in U.S. Pat. No. 4,747,160) employs omni-directional polarized antennas, each antenna broadcasting into a circular service area. Each antenna transmits signals having either a horizontal or vertical polarity. Subscribers in this prior art system use directional antennas that are tuned to a given transmission polarity and physically aimed towards a transmitter tower having the corresponding polarity. As a result, interference from an adjacent transmitter tower is eliminated if the adjacent tower is transmitting a cross-polarized signal. However, the problem with this transmission scheme is that only two polarizations are available, but there exist some subscriber locations which will be illuminated by transmissions from three or more towers. At least one of these additional interfering signals will be of the same polarity as the subscriber's antenna. Consequently, at least one interfering signal may enter the subscriber's antenna along with the desired signal, and the quality of the desired signal may be degraded substantially.
The present invention is a digitally implemented multichannel data distribution system and method that overcomes the above described problems. Specifically, the system of the present invention employs digital signal processing techniques to combine the analog television channel signals (i.e., the audio and video signal components) and other channel signal types (such as digital television signals, teleconferencing signals, interactive programming signals, computer data signals, and video-on-demand signals) into a single stream of formatted data. Then, the system of the present invention uses special modulation methods to reduce the effective spectrum bandwidth of the transmitted signal. As a result, the present invention is able to fit many more independent channels into an authorized operating spectrum bandwidth than can the prior art transmission systems thereby overcoming the prior art limitation of obtaining many individual and independent channels required to implement narrowcasting.
The system and method of the present invention also employs a multifaced sectorized antenna at each of the transmitter sites 11. The sectorized antenna, comprised of a plurality of independent smaller antennas, essentially divides each service area into a plurality of independent wedge-shaped azimuthal sectors. For each azimuthal sector, independent channel signals and other data are received from the provider studio as a single digital data stream which is modulated, amplified, and transmitted to the appropriate subscribers residing in that specific sector. The signals transmitted into the other azimuthal sectors surrounding the transmitter site are independent of one another and contain different data that is of interest to the subscribers in those other sectors. Furthermore, the antenna polarities of adjacent sectors are of opposite polarities. Therefore, the RF signals from the two sector antennas cannot combine destructively, and the subscriber antenna will receive only one of the two possible sector signals.
The current invention can be implemented either to provide only one-way wideband transmission from the provider site through the transmitter sites to the subscriber sites, or it can be implemented to provide two-way wideband transmissions between the provider site and the subscriber sites via the transmitter sites.
In the preferred embodiment, the two-way wideband transmission process would be used for video conferencing between subscribers. Consequently, the provider site would not be the final destination for the transmission from the subscribers, but it would provide switching service to interconnect the two-way transmissions among the subscribers who are participating in the video conference. Alternatively, the switching function could also be provided at each transmission tower, thereby eliminating the need to route all signals back to the provider site.